It's because he was the air element, so he gets the brash independent guy stereotype. Wait, he got turned evil twice? I only remember him getting the corroded mask thing once. Or are you talking about something after the initial plot? Because I stopped caring at that point, since the silver armor was shitty looking and they ran out of cool mask designs.
Also, Kopaku got so gypped in the powers department. Never thought there'd be a power less useful than "swim fast, talk to fish", but he ROUTINELY made Gali look useful by comparison.
Also also, why do I remember all these people's names? It's been years since I so much as looked at them.